1. Field of the Invention
The present invention relates to a thin-film magnetic head, and a method of manufacturing the magnetic head. More particularly, it relates to a method of polishing a surface to-be-worked made up of different kinds of materials having unequal hardness values, such as the air bearing surface or floating surface of a thin-film magnetic head. Especially, it relates to a method of manufacturing a thin-film magnetic head so as to be able to protrude the end face of a magnetic film beyond the air bearing surface of the head.
2. Description of the Related Art
In recent years, as an expedient for enhancing the record density of a magnetic record medium in a magnetic recording/reproducing system, research has been carried out on the approach of a magnetic head to the magnetic record medium. FIG. 3 illustrates the positional relationship between the magnetic head and the magnetic record medium in the magnetic recording/reproducing system in the prior art. As clearly shown in FIG. 3, the air bearing surface 2 of the thin-film magnetic head 1 is afloat above the magnetic record medium 21 by a certain flying height 29. In this regard, it has been known that a recorded bit length on the magnetic record medium 21 is in a proportional relationship with the flying height 29 of the magnetic head 1 as illustrated in FIG. 4. In the case of FIG. 4 mentioned by way of example, the bit length increases 0.05 [.mu.m] as the flying height 29 increases 0.01 [.mu.m]. For the increase of the record density, therefore, it is desired to decrease the flying height 29 of the magnetic head 1 as much as possible.
Meanwhile, the thin-film magnetic head 1 includes a protective film 12 and a magnetic film 11 which are stacked on a substrate 13 as illustrated in FIGS. 2A and 2B and FIG. 3. The air bearing surface 2 of the magnetic head 1 is formed by so-called "lapping". Lapping is a method wherein, while a lapping fluid which contains diamond abrasive grains is being dropped onto a lap (lapping plate) made of a soft metal, the workpiece (the magnetic head 1 as a semi-finished product) held in position by a holder is slid under pressure so as to polish and form the air bearing surface 2 of the magnetic head 1. The lapping method is stated in, for example, "TRIGGER 88-1 (1988)" published by Kabushiki-Kaisha Nikkan Kogyo Shinbun-sha, p. 51.
Besides, as disclosed in the official gazette of Japanese Patent Application Laid-open No. 178910/1992, there has been proposed a method wherein the air bearing surface of a thin-film magnetic head is formed so as to protrude a magnetic film beyond this air bearing surface by so-called "float polishing" which polishes the workpiece (the magnetic head as a semi-finished product) without pressing abrasive grains against the workpiece.
All of the prior-art techniques, however, have been problematic as stated below.
When the surface of the workpiece made up of the plurality of members or constituents of the substrate 13, protective film 12 and magnetic film 11 having hardness values unequal to one another, such as the air bearing surface 2 of the thin-film magnetic head 1, is polished by the conventional lapping method utilizing the diamond abrasive grains, there arises the problem that a pole tip recession 22 appears in the worked or finished surface as shown in FIG. 2B and FIG. 3.
The appearance of the pole tip recession 22 is attributed to the fact that, while the workpiece made up of the plurality of members having the unequal hardness values is subject to the finish working under an identical lapping pressure, the intrusional depths of the abrasive grains (namely, the amounts to which the abrasive grains bite into the members) become different due to the differences of the hardness values of the materials of the individual members. Generally in the thin-film magnetic head, the material of the magnetic film (11 in FIG. 2B or FIG. 3) is softer than those of the protective film (12) and substrate (13). By way of example, the thin-film magnetic head 1 in the prior art is constructed of the magnetic film 11 whose Vickers' hardness number is about 200 [kgf/mm.sup.2 ], and the protective film 12 and substrate 13 whose Vickers' hardness numbers are 1300 [kgf/mm.sup.2 ] or above. In this magnetic head 1, the pole tip recession 22 having appeared between the magnetic film 11 and the substrate 13 is in the order of 0.03 [.mu.m].
As seen from FIG. 3, when such a pole tip recession 22 exists, the magnetic film 11 cannot approach the record medium 21 in correspondence with the recession 22, and hence, the flying height 29 of the thin-film magnetic head 1 cannot be, in effect, reduced. More specifically, in the example mentioned in FIG. 4, when the pole tip recession of 0.03 [.mu.m] exists in the air bearing surface of the magnetic head, the bit length becomes 0.15 [.mu.m] greater than in the nonexistence of this recession, and the record density of the magnetic record medium (21 in FIG. 3) is lowered correspondingly.
On the other hand, according to the float polishing method as disclosed in the official gazette of Japanese Patent Application Laid-open No. 178910/1992, the magnetic film can be protruded beyond the air bearing surface, whereby the flying height of the magnetic head can be decreased. Since, however, the float polishing method does not press the abrasive grains against the surface to-be-worked or workpiece, it has the problems of a low abrasion working rate and a low production efficiency. Moreover, the float polishing method features that the edges of the air bearing surface are worked more efficiently. This leads to the problem that the edges of the air bearing surface of the magnetic head are abrasively rounded, so that it is difficult to form a protective film of carbon, silicon or the like at a uniform thickness on the air bearing surface.